The invention relates to athletic playing surfaces and more particularly to an athletic track having a post-tensioned concrete foundation.
Track and field sport competitions are frequently held on athletic tracks. Such tracks usually have a standard shape with opposed straight sides connected by semi-circular ends. A foundation usually supports an athletic surface. The track should be durable yet resilient to aid athletic performance and minimize injury. Originally, cinder tracks and other loose aggregate surfaces were used. More recently composite surfaces have been employed. Composite surfaces have a foundation, often poured-in-place concrete, and an athletic surface, often a resilient composite such as asphalt, a polymeric substance, or a mixture of materials. In any case, it is important that the track not develop cracks which could cause injuries. The track should also have a configuration which sheds water easily, particularly if the track is outdoors.
The development of cracks in concrete foundations is a well-known phenomenon. Ground-supported, reinforced concrete slabs, commonly referred to as xe2x80x9cfloating slabsxe2x80x9d, began to be used in residential and commercial construction after about 1945. Although floating slabs worked very well in many cases, they developed unacceptable cracks when used on xe2x80x9cexpansivexe2x80x9d soils. Expansive soils absorb water and swell during rainy conditions, but shrink and dry during hot, dry weather. The combination of absorbent soils and a climate characterized by extended dry periods interspersed with significant rain can be found in many parts of the United States and particularly in California, Texas and Louisiana. During the mid 1960""s, post-tensioned concrete slab foundations began to be used to support residential buildings in parts of Louisiana, Texas and California. Such slabs are called xe2x80x9cpost-tensionedxe2x80x9d because they are stressed in tension after the concrete has set. A post-tensioned concrete slab comprises a poured-in-place concrete slab with metal strands or cables imbedded in the slab. The cables protrude from opposed parallel edges of the slab. Generally, there are two sets of parallel cables at right angles to each other, allowing the slab to be stressed in orthogonal directions. After the concrete has been poured and has set, the cables are pulled to a preselected tension, putting the slab under a compressive load. Concrete is very strong in compression, but weak in tension. A permanent compressive load on the concrete helps the slab resist cracking as the supporting ground swells and shrinks with changing water conditions.
Cracking occurs as the moisture content of the soil beneath a foundation changes differentially across the foundation. For example, the slab may be distorted in either a center lift mode (also termed xe2x80x9cedge drying,xe2x80x9d xe2x80x9ccenter heavexe2x80x9d, or xe2x80x9cdomingxe2x80x9d) or an edge lift mode (also, xe2x80x9cedge swellxe2x80x9d, xe2x80x9cedge heavexe2x80x9d or xe2x80x9cdishingxe2x80x9d). Center lift mode is a long-term condition which occurs when the moisture content of the soil around the slab edges decreases and the soil sinks relative to the soil beneath the center of the slab. Edge lift mode is usually a seasonal, short-term condition which occurs when the ground at the edge of the slab becomes wetter than the ground beneath the center of the slab. The distance measured inward from the edge of a slab over which the moisture content varies is known as the xe2x80x9cedge moisture variation distancexe2x80x9d or xe2x80x9cedge penetration distancexe2x80x9d. The edge penetration distance is generally a function of climate. Wetter climates create larger edge penetration distances, and potentially greater bending in either center lift or edge lift modes.
Athletic tracks with concrete foundations are subject to the same kinds of changes experienced by residential or commercial foundations. Moreover, the extended shape of the track may present additional problems because a track is relatively long and narrow. The curved ends also present peculiar problems for stressing a concrete slab.
There remains a need, therefore, for an athletic track having a post-tensioned concrete slab foundation. An object of the present invention is to provide such an athletic track.
The present invention comprises an athletic track having a post-tensioned concrete slab supporting a resilient surface. The athletic track has opposed, parallel rectangular side areas and opposed semi-circular end areas connecting the side areas. The concrete slab is cast in place without requiring expansion joints between segments. Cables are cast into the slab for providing a tension load in the concrete slab. The cables are not perpendicular to exposed sides of the rectangular side areas. A first set of cables is tilted away from the perpendicular in a first direction while a second set of cables is parallel to the edges of the rectangular areas such that members of the second set of cables cross members of the first set of cables. In another aspect of the invention, a third set of circumferential cables is provided in the semi-circular end areas. A fourth set of cables intersects the circumferential cables. The fourth set of cables is preferably non-radially disposed in the semi-circular end area. In another aspect of the invention, the slabs forming the track have a relatively flat lower surface, without downwardly extending footings. In a further aspect of the invention, pour strips are provided between adjacent quadrants of the end areas. In another aspect, the pour strips are situated along a line of symmetry.
It is an object of the invention, therefore, to provide an athletic track with a substrate of post-tensioned concrete.
It is a further object of the invention to provide an athletic track with a concrete substrate without features extending from a bottom surface thereof for use on expansive soils.
It is another object of the invention to provide an athletic track with a concrete substrate with increased longitudinal compression.
It is also an object of the invention to provide pour strips for a post-tensioned athletic track wherein the stresses on the pour strips are reduced.
Another important object is to provide a stable substrate for an athletic track to increase the longevity of a resilient surface on the substrate.
Yet another object of the invention is to provide a method of constructing an athletic track having a post-tensioned concrete substrate.
These and other features and objects of the invention will be apparent from the following detailed description, in conjunction with the accompanying drawings.